1. Field of the Invention
This invention relates generally to burglar alarms and more particularly to electronic burglar alarms for automobiles and other vehicles.
2. Description of the Prior Art
The prior art teaches many types of vehicle burglar alarms. Typically these alarms are manually armed by a hidden or locking switch, and are disarmed before or shortly after re-entry into the vehicle.
A problem with many of these prior art vehicle burglar alarms is that they must be manually armed and disarmed. If, for example, a driver were to forget to arm the burglar alarm his or her vehicle would be left unprotected. If the driver remembered to arm the alarm but forgot to disarm it, he or she would be caught in the embarrassing position of setting off their own vehicle's alarm system.
A number of prior art patents have addressed this problem by automating the arming of a burglar alarm. For example, in U.S. Pat. No. 4,159,466 of Mengel a burglar alarm system is described which utilizes a flip flop (IC5) to start or stop a clock (IC1). As noted in column two of the description, a voltage developed by the opening of a vehicle's door on pin 6 of IC5 causes the flip flop to change state and start the clock timer. Thus, Mengel's alarm is armed in response to the opening of a vehicle's door. A problem with Mengel's device is that the burglar alarm is armed and will go off after a period of time even if the door is left deliberately open, such as for the unloading of cargo from the vehicle.
In U.S. Pat. No. 4,262,279 Dublirer teaches an alarm which "automatically provides . . . protection without requiring the owner to perform any [special] acts . . . ". The invention of Dublirer has the same disadvantage pointed out with Mengel, i.e. the alarm is armed with the opening of the vehicle's door and not with its subsequent closure.
Gurgone in U.S. Pat. No. 4,123,745 describes two embodiments of an auto alarm which disables the ignition after a door is opened. He thereby circumvents the problem of Mengel's and Dublirer's inventions because no alarm will sound if the vehicle's door is left open for extended periods of time. Unfortunately, this circumvention precludes the use of an audible alarm to broadcast the fact of an illegal entry into the vehicle.
A number of other patents teach the use of delay circuits to allow a vehicle's operator to enter or exit the vehicle without setting off the alarm. Typical examples of such are U.S. Pat. Nos. 4,188,621 and 4,136,334. The prior art burglar alarms with delay circuits are often complex and expensive, and are not always capable of complex functions.